The present invention relates to glass, more specifically to novel glasses, novel glass compositions and to substrates composed of the novel glass compositions. The novel glass compositions are soda lime silica glasses, but possess relatively high strain points compared with known soda lime silica glasses, in particular those soda lime silica glasses which are in common use to produce flat glass by the float process. Consequently, the novel glasses of the invention are suited to applications requiring good dimensional stability at high temperatures, such as fire protection glazings and substrates for processing at high temperature. The substrates are suitable for the deposition of coatings and the manufacture of display panels, discs, e.g. magnetic recording discs, semiconductor devices, including photovoltaic cells, especially solar cells, amongst other applications.
While normal soda lime silica glass (i.e. soda lime silica glass compositions in common use for windows and other glazings for buildings and vehicles) has suitable properties at room temperature for many of the above applications, the applications may require processing of the glass at high temperatures, i.e. at temperatures which are above the strain point, or the annealing point, or even the softening point of the glass. Processing the glass at such elevated temperatures would result in the creation of permanent internal stresses in the glass, possibly leading to distortion or fracture of the glass. The glass may even become distorted or deformed during processing. Attempts have therefore been made to provide glasses which are more suited to high temperature processing, i.e. which possess improved high temperature stability by virtue of having relatively high strain points.
The strain point (which is defined as the temperature at which the viscosity of the glass is 1014.5 poise, denoted T log 14.5 poise) of soda lime float glass in common use is in the region of about 510° C. to 540° C., depending on the precise composition. However, many glasses with higher strain points are known. One group of glasses with high strain points is the so-called alkali-free glasses. Unfortunately, these glasses are difficult and expensive to produce, owing to the lack of alkali which acts as a flux. Many of these glasses are also unsuitable for forming by the float process. The use of alternative forming processes generally adds further to the cost, and may result in inferior flatness or smoothness of the surface. A further problem with alkali-free glasses is that they tend to have very low coefficients of thermal expansion, which renders them unsuitable for some applications.
Another group of glasses with relatively high strain points contains increased potash and reduced soda, compared with common or “normal” soda lime silica float glass.
TiO2 
Unfortunately, high potash glasses are themselves difficult to produce in the open regenerative furnaces generally employed in float plants, because high potash glasses may be difficult to refine in such furnaces.
A number of attempts have been made to provide a soda lime silica glass composition having a higher strain point than common float glass. U.S. Pat. No. 5,599,754 discloses a glass composition for a substrate, which is useful for flat display panels, particularly for plasma display panels. The claimed compositions contain from 6 to 9% SrO, which is expensive, and adds significantly to the cost of raw materials when used at these relatively high levels.
U.S. Pat. No. 6,905,991 is an example of a soda lime silica glass composition containing relatively low levels of soda (from 2 to 8% Na2O) and relatively high levels of potash (from 0-8%, but all of the Examples contain at least 3.5% K2O). The resulting glasses may be used for producing fireproof glazing panels or for substrates for display panels.
WO 98/49111 discloses a glass composition for a plasma display panel, the glass having a lower density than previous glasses for plasma display panels. The total amount of BaO and SrO included in the total alkaline earth metal oxides in the glass lies in the range of 1 to 8%. Again, these oxides are expensive.
U.S. Pat. No. 6,087,284 relates to an aluminosilicate glass which is suitable for use in display technology. This patent seeks to find a glass which has a high transition temperature, low density and is solarisation-stable. Preferably the glass contains MgO in trace levels at most, or not at all. It therefore represents a substantial departure from the composition of normal or common float glass.
U.S. Pat. No. 7,273,668 relates to a glass composition having high heat resistance which is suitable for chemical strengthening. The composition may be used to produce glass substrates for magnetic recording media, e.g. hard disk drives. Unfortunately this glass is prone to devitrification, which can make manufacturing difficult, and reduce yields.
KR 2009 0111680 A discloses a glass composition for display panels, which seeks to improve reactivity and failure rate of electrode patterns.
JP 2010 143790 A discloses a method for producing a glass substrate for a solar cell in which waste glass can be efficiently recycled. The waste glass is used as part of the glass raw material, which is then melted in a glass melting furnace and formed into the glass substrate.
U.S. Pat. No. 8,828,897 relates to aluminosilicate glasses having high thermal stability and low processing temperatures. The glasses may be used as substrate glass, superstrate glass and/or cover glass for photovoltaic applications and other solar technology applications.
U.S. Pat. No. 8,895,463 relates to a glass substrate for a solar cell such as a Cu—In—Ga—Se (“CIGS) solar cell. The glass compositions of the invention deviate significantly from common soda lime silica glass, being low in soda and high in potash.
US 2013/0306145 A1 also relates to a glass substrate for a CIGS solar cell, and again, the glass compositions are low in soda and high in potash.
US 2013/0313671 A1 relates to glass substrates for solar cells, such as CdTe or CIGS cells. It is stated that the content of SrO, BaO, B2O3 and/or ZrO2 is advantageously zero in order not to penalize the cost of the glass sheet. However, judging by the Examples provided, this approach yields only modest increases in strain point compared with common soda lime silica glass.